1. Field of the Invention
The present invention relates to time synchronization in a network, and more particularly, to a method of performing precise timestamping of transmission and receiving time values of a synchronization signal to improve precision of frequency synchronization and time (time of day, TOD) synchronization at a transmission/receiving terminal of a baseband communication network or in asynchronous modulation/demodulation when the transmission terminal and the receiving terminal are separated.
The present invention is supported by the Information Technology (IT) Research & Development (R&D) program of the Ministry of Knowledge and Economy (MKE) and the Institute for Information Technology Advancement (IITA) [2007-S-012-02, Development of Multimedia Convergence Network On-chip Technology].
2. Description of the Related Art
In general, in a packet switched network, a protocol for time synchronization is needed to synchronize time of systems distributed throughout the network. A system providing a reference time for the time synchronization is set as a master and a system subject to the time synchronization with the master is set as a slave. As the master and the slave exchange messages or sync signals including time information, the slave may be time synchronized with the master.
Of the protocols for the time synchronization between the master and the slave connected to the network, a network time protocol (NTP) is one of the previously used Internet protocols. The NTP is a time synchronization method that is currently widely used for the fields of local area network (LAN) and wide area network (WAN). The NTP synchronizes computers connected to the network using the coordinated universal time (UTC) that is an international standard time. Particularly, since additional hardware is not needed, the NTP is cost effective and is precise to a degree of tens to several tens of milliseconds in a normal Internet condition.
However, in the applied fields requiring a precise time synchronization, for example, in the field of multimedia streaming services in the packet switched network, due to a limit in the precision of the NTP, there is a demand for a time synchronization protocol having a higher precision level. To address this matter, IEEE has developed and standardized a precision time protocol (PTP) having an improved precision level.
FIG. 1 illustrates one of the methods of performing time synchronization between the master and the slave in the PTP. The PTP performs time synchronization as the master and the slave exchange time information and a message or sync signal related to the time information in a manner similar to the NTP. Referring to FIG. 1, while exchanging the message or sync signal, the master and the slave determine an offset between the clock of the master and the clock of the slave and a propagation delay time of the message transmitted through a network so that the clock of the slave is synchronized to the clock of the master.
The master and the slave measure a start or arrival time of a signal by exchanging sync signals, for example, SYNC, REQ, and RESP, and then calculate the offset and the propagation delay time. As shown in FIG. 1, after the final time synchronization process, the slave has the time information of t1, t2, t3, and t4 and an offset O and a propagation delay time D are calculated using the time information and the following Equations 1-4.D+O=t2−t1   [Equation 1]D−O=t4−t3   [Equation 2]D={(t2−t1)+(t4−t3)}/2   [Equation 3]D={(t2−t1)−(t4−t3)}/2   [Equation 4]
As a condition to satisfy the above Equations, it is assumed that a propagation delay time DMS to transmit a signal from the master to the slave and a propagation delay time DSM to transmit a signal from the slave to the master are symmetrically the same. Unlike the NTP, the PTP uses an auxiliary hardware to measure a timestamp value corresponding to the transmission and receiving times of a sync signal and uses the measured timestamp value for the calculation of the offset and the propagation delay time so that precision of time synchronization may be lowered to a degree of microseconds or less.
In order to use the above conventional technology to an applied field that requires a high time synchronization precision, the following problem need to be solved first.
According to the conventional technology, while the master and the slave exchange a sync signal, a start time when the sync signal starts from a transmission terminal for transmitting the sync signal, or an arrival time when the sync signal arrives at the receiving terminal for receiving the sync signal, is measured. In doing so, timestamp values are measured using a local clock at each of the transmission terminal and the receiving terminal to be used as the timestamp values. Since the local clock frequencies of the transmission terminal and the receiving terminal are different from each other, an error is generated between an precise arrival time and an actually measured time value. As a result, the accuracies of a timestamp value of the start time of the sync signal measured at the transmission terminal and a timestamp value of the arrival time of the sync signal measured at the receiving terminal are deteriorated. If a measurement error is generated in the start time and arrival time of the sync signal, an error may be accordingly generated in the offset and the propagation delay time calculated at the slave. Therefore, the precision of the time synchronization may be deteriorated.